Many industrial bonding processes use slow-curing two-component adhesives (about 24 hours to final strength) or heat-curing one-component adhesives (about 0.5 to 1.5 hours to final strength, without counting the time for heating the components). When the focus is on reducing cycle times, light-curing products are primarily considered. Some of these products cure in less than one second.
In the past, at least one of the components to be joined had to be optically transparent, allowing light energy to reach the adhesive and trigger the curing mechanism. This reduced the choice of materials to a few types of plastic and glass. Today, new adhesive chemistries paired with multi-stage curing processes allow cycle times of many other bonding processes, for materials like metals and opaque plastics, to be shortened significantly.
Light Fixation for Structural Bonding
If structural bonds are exposed to greater static dynamic loads or increased temperatures, purely heat-curing epoxy resin adhesives should be used. However, such products have not been available with light-curing properties. As a result, fixing devices were frequently used to hold in position the components in the production lines and during oven curing.
Recently, light-fixable products have become available for such applications. Their two-stage light and heat-curing mechanism simplifies and accelerates the production process. The joined components are first prefixed at the adhesive fillet, which takes one to five seconds depending on the intensity of the UV light. This eliminates the need for holding devices along with time-consuming and costly assembly, disassembly and cleaning. The epoxy resin reaches its full strength with the subsequent, and still necessary, step of heat curing, which usually takes 20 minutes at 130 °C or even less at higher temperatures.
|Light-fixable structural adhesives are used, for example, in the production of electric motors (Image source: DELO)|
The final strength on aluminum is 60 MPa, on PA6 it is 30 MPa. 60 MPa correspond to a force of 1.2 tons on the surface of a penny. In addition, they have similarly high thermal and chemical resistance as purely heat-curing adhesives.
Using the Diversity of Dual-Curing Adhesives
When two components are bonded, it is important that all the adhesive is fully cured. If the light reaches only part of the adhesive, it will remain liquid in the so-called shadowed areas. On the one hand, no adhesion is built up in those areas. On the other hand, there is a risk of corrosion. If speed requirements suggest the use of light-curing adhesives, shadowed areas should be avoided starting from the design stage.
For the many cases where this is not possible or possible only with great difficulty, there are numerous new dual-curing adhesives that reliably build up adhesion even in shadowed areas. In addition to light, they feature a second curing mechanism that can be triggered by air humidity, exclusion of oxygen or heat. Each option meets different requirements and opens up other production processes.
Dual-curing adhesives offer the benefits of light-curing products without compromising on reliability, bond strength and processing quality. They also ensure that the adhesive in the finished product is fully cured and permit maximum bonding precision in complex modules. They offer a high degree of flexibility in production, giving users more freedom in designing assemblies and developing their production processes.
Light Curing for Black Adhesives
If light-curing products are used, it goes without saying that the adhesive itself must also be translucent, since the photoinitiators in the entire adhesive layer must decompose to start the crosslinking reaction. Therefore, light curing in combination with black encapsulants and adhesives often used for optical purposes or security reasons should, by definition, be a contradiction, because black absorbs most of the light. But it’s not.
There are black adhesives that cure very well in layer thicknesses of up to 500 µm. In addition to their light-curing component, they also contain a small humidity-curing portion, which triggers crosslinking even in shadowed areas. These products typically provide good strength. The flexibility offered by their acrylate chemistry, exhibits high elongation at tear and very good tension-equalizing properties.
|Even black adhesives can be light-cured (Image source: DELO)|
Preactivation: Light Curing Even for Opaque Components
If opaque materials are to be bonded and there is no fillet weld that can be reached by light, as is often the case in the automotive industry that uses decorative elements made of dark plastics or chrome, light curing alone is impossible due to the opacity. However, users may use adhesives that can be preactivated by light in order to bond non-transparent components and still benefit from rapid curing. A conventional light curing procedure includes dispensing, joining and irradiation. In contrast, when using a preactivation process, the complete adhesive is applied to one side of the two components and is irradiated with light immediately before joining the components.
The special feature of this process is that the adhesive still remains liquid after the brief exposure to light, allowing the components to be joined and adjusted during a period called “open time,” after which the adhesive cures within a few minutes without further light irradiation.
|In contrast to the conventional light-curing steps of dispensing, joining, and irradiating, the order for preactivation is dispensing, irradiating, and joining (Image source: DELO)|
High-Intensity LED Lamps
LED technology has continued to evolve in recent years, especially with regard to the intensities achieved. Good area curing lamps now offer values of more than 1,000 mW/cm² at a distance of 2 mm. In most cases, conversion to modern, high-intensity LED lamps can significantly accelerate the light-curing process, since more energy reaches the adhesive.
This is advantageous even if the maximum intensity of the adhesive – the threshold from which higher intensity no longer leads to faster curing – is lower than the lamp's intensity. This applies to components with poor transmittance, which absorb light energy and prevent the full intensity from reaching the adhesive.
|High-intensity LED curing lamps not only accelerate curing, but also provide more flexibility in production (Image source: DELO)|
In addition, the use of high-intensity curing lamps allows larger exposure distances, which is useful for difficult geometries such as holes or when the joined materials are a little further from the light source due the component design or the assembly line layout. In these cases too, sufficient energy is available thanks to the lamp’s high intensity levels, and the adhesive cures within seconds.
Even when full intensity is not needed, there is an advantage: Users may reduce energization of the powerful lamps. This protects the LEDs and extends their already very long lifetime of typically 20,000 hours.
Bernd Scholl is the head of product technology at DELO.